This application relates to a process for producing an electroluminescent phosphor, and particularly, to a process that produces an electroluminescent phosphor that has increased efficiency.
Electroluminescent phosphors are used for backlighting in LCD""s, in copying machines, for backlighting membrane switches, for automotive dashboard and control switch illumination, for automotive interior lighting, for aircraft style information panes, for aircraft information lighting, and for emergency egress lighting. U.S. Pat. Nos. 3,014,873; 3,076,767; 4,859,361; 5,009,808; and 5,100,499 relate to methods for producing electroluminescent phosphors. The methods described in these patents result in electroluminescent phosphors that have achieved general commercial success; however, the efficiency of these phosphors has left something to be desired. The present process results in a phosphor that requires less power, allowing it to be used in smaller devices where power consumption is a concern. It would, therefore, be an advance in the art to provide phosphors having increased efficiency. Efficiency is defined as light output per unit of power consumption and as used herein the term shall comprise lumens per watt or LPW.
It is, therefore, an object of the present invention to obviate the disadvantages of the prior art.
It is another object of the invention to produce electroluminescent phosphors that are more efficient than those heretofore produced.
These objects are achieved, in one aspect of the invention, by synthesizing the phosphor with Ga2O3. The reaction takes place under the following conditions; zinc sulfide, in the presence of activators and halide fluxes, is heated in a furnace to produce a phosphor material. This material is cooled and washed. This material is known as first step fired material (FSF). The phosphor at this stage is largely inactive with respect to electroluminescent output. The FSF material is then blended with other materials, including Ga2O3, and then heated to a lower temperature than the first step firing. The resulting material is then cooled, washed again and sifted, creating a new electroluminescent phosphor having an efficiency greater than the original phosphor.
For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims.
The increase in efficiency of the electroluminescent phosphors gained by the practice of the process is shown by the examples below:
In Table I, the color coordinates for the phosphors are given per the standard C.I.E. Chromaticity Diagram; the brightness is at the 2 hour and 24 hour periods of lamp operation and the efficiency, as noted, is in lumens per watt.
As can be seen from Table I, when the electroluminescent phosphors prepared by the process herein disclosed are compared to the non-treated phosphor, the phosphor synthesized with Ga2O3 has an increased efficiency with virtually no change in color coordinates.
More specifically, the process is especially suited for zinc sulfide, copper activated phosphors or other zinc sulfide phosphors where copper is a co-activator.
The method comprises heating zinc sulfide (approximately 1% chloride-containing) in a furnace to an elevated temperature (e.g., 1205xc2x0 C.) in the presence of a copper activator and halide fluxes (as is known in the art) to provide a phosphor that is substantially inactive with respect to electroluminescent activity. This phosphor is then cooled to ambient temperature and washed to remove the flux. The resulting ZnS:Cu,Cl phosphor is then milled, remixed with Cu compounds and ZnSO4 and refired. The refired materials are then washed, dried and sifted. Phosphors created by this method are known. One such phosphor is a Type 723 electroluminescent phosphor available from Osram Sylvania Inc., Towanda, Pa.
The method of the invention is practiced by blending a FSF ZnS:Cu,Cl phosphor with other materials, including Ga2O3. In a preferred embodiment, the other materials comprise CuSO4, ZnSO4.7H2O and sulfur; however, other materials may be used, as known in the art. This mixture is placed in a first inert vessel, such as a plastic bottle. The mixture is blended for a period of time, e.g., 20 minutes to 30 minutes, by mechanical shaking or other similar method. The blended mixture is then loaded into a second inert reaction vessel, for example, 100 ml alumina crucibles. The second reaction vessel is then fired for a second period of time. The second step fired cake is then cooled and washed with de-ionized water. Preferably, 1.242 liters of water are used per 75 grams of FSF material used in the 2nd step blended mixture. The mixture is then washed with acetic acid to eliminate some excess copper and zinc compounds and other impurities. In a preferred embodiment, 0.777 liters of hot de-ionized water+148.8 ml glacial acetic acid per every 75 grams of the fired SSM was used but other concentrations are useable. The material is then washed again with de-ionized water to wash off any residual acid. The acid-free material is then washed with KCN solution to eliminate any remaining unwanted copper. In a preferred embodiment, 0.777 liters of hot de-ionized water+37.54 grams of KCN per 75 grams of the SSM was employed. The material was allowed to cool and washed with de-ionized water to remove residual KCN. The resulting phosphor is then dried, allowed to cool and sifted through a xe2x88x92325 mesh stainless steel sieve.